Graded glass/zirconia/glass structures for damage resistant ceramic dental and orthopedic prostheses

ABSTRACT

The present invention provides a functionally graded glass/ceramic/glass sandwich system for use in damage resistant, ceramic and orthopedic prosthesis. The functionally graded glass/substrate/glass composite structure comprises an outer residual glass layer, a graded glass-ceramic layer, and a dense interior ceramic. The functionally graded glass/substrate/glass composite structure may further comprise a veneer on an exterior surface. The present invention also provides a method for preparing a functionally graded glass/ceramic/glass sandwich system. A powdered glass-ceramic composition is applied to the accessible surfaces of a presintered zirconia substrate to thereby substantially cover the substrate surfaces. The glass of the composition has a CTE similar to that of the substrate material. The glass-ceramic composition is infiltrated into and densifies the substrate by heating the assembly to at least the sintering temperature of the substrate.

FIELD OF THE INVENTION

The present invention relates to dental and orthopedic prostheses andmethods for producing improvements in dental and orthopedic prosthesesusing functionally graded materials (“FGMs”) such as a functionallygraded glass/zirconia/glass (G/Z/G) sandwich material.

BACKGROUND OF THE INVENTION

Teeth play a critically important role in our lives. Loss of functionreduces the ability to eat a balanced diet which results in negativeconsequences for systemic health. Loss of aesthetics can negativelyimpact social function. Both function and aesthetics can be restoredwith dental crowns and bridges. Ceramics are attractive dentalrestoration materials because of their aesthetics, inertness, andbiocompatibility. However, ceramics are brittle and subject to prematurefailure, especially after repeated contact including slide-liftoffmasticatory loading in a moist environment (Kim et al. (2007) Journal ofDental Research 86(11): 1046-1050; Lawn et al. (2001) The Journal ofProsthetic Dentistry 86(5): 495-510; Lawn et al. (2001) J Prosthet Dent86(5): 495-510; Zhang et al. (in press) “Fatigue Damage in CeramicCoatings from Cyclic Contact Loading with Tangential Component.” Journalof the American Ceramic Society) Fracture rates of ceramic restorationsmay seem low at 3-4% per year (Fradeani et al. (1997) Int. J.Prothodont. 10: 241-7; Malament et al. (1999) J. Prosthet. Dent. 81:23-32; Sjogren et al. (1999) Int. J. Prosthodont. 12: 122-8; Sailer etal. (2006) Quintessence International 37(9): 685-693; Sailer et al.(2007) Clin. Oral Impl. Res. 18(3): 86-96; Pjetursson et al. (2007)Clin. Oral Impl. Res. 18(3): 73-85). However, failure can causesignificant patient discomfort and loss of productive lifestyle. Thevulnerability of dental ceramic restorations is exacerbated by damage,fatigue loading, and moisture.

According to a survey conducted by American Dental Association, morethan 45 million new dental crowns, of which over 37 million wereporcelain (ceramic) based, were provided by dentists in 1999 (ADA(2002). “The 1999 Survey of Dental Services Rendered.”). As thepopulation ages, the number will increase. Despite continuous efforts toimprove the strength of dental ceramics, all-ceramic dental crownscontinue to fail at a rate of approximately 3-4% each year (Burke et al.(2002) J Adhes Dent 4(1): 7-22). The highest fracture rates are onposterior crowns and bridges where stresses are greatest. Dental crownsgenerate over $2 billion each year in revenues with 20% of the unitsbeing all-ceramic (Nobel Biocare 2004). Dental ceramics that best mimicthe optical properties of enamel and dentin are predominantly glassymaterials principally feldspar (a group of minerals having mainconstituents of silica and alumina) (Kelly (1997) Annual Reviews ofMaterials Science 27: 443-68; Kelly (2004) Dent. Clin. N. Am. 48:513-30). The original dental porcelain contained high feldspathic glasscontent and was extremely brittle and weak (S (strength) approximately˜60 PMa) (McLean, J. W. (1979) The Science and Art of Dental Ceramics.Chicago, Quintessence Publishing Co. Inc.; Binns, D. (1983) The Chemicaland Physical Properties of Dental Porcelain. Chicago, QuintessencePublishing Co. Inc.). Therefore, despite the aesthetic advantage, theearly porcelain crowns were not widely used in dentistry (Van, N. R.(2002). “An Introduction to Dental Materials.” 231-46).

Dental ceramics have become increasingly popular as restorativematerials due to improvements in strength. Several methods have beendeveloped to improve the strength of dental ceramics including addinguniformly disperse appropriate filler particles throughout a glassmatrix, referred to as “dispersion strengthening” (McLean et al. (1965)Br. Dent. J. 119: 251-67). The first fillers used in dental ceramicswere leucite particles (Denry (1996) Crit. Rev. Oral. Biol. Med. 7:134-43). Commercial dental ceramics containing leucite as a dispersionstrengthening fillers include IPS Empress (S˜120 PMa) (Ivoclar-Vivadent,Schaan, Liechtenstein) and Finesse All-ceramic (S approximately 125 MPa)(Dentsply Prosthetics, York, Pa.). Particle strengthening can also beachieved by heat-treating the glass to facilitate the precipitation andsubsequent growth of crystallites within the glass, termed “ceraming”.Dental ceramics produced using the ceraming process are calledglass-ceramics. Several commercial products such as Dicor (S˜160 MPa)(Dentsply), IPS Empress II (S˜350 MPa) (Ivoclar-Vivadent) and, morerecently, IPS e.max Press (S˜525 MPa) (Ivoclar-Vivadent) are examples.The leucite-strengthened porcelains and the glass-ceramics aretranslucent, so single layer (monolithic) crowns can be made from thesematerials. However, only moderate strength increases can be achieved viathe particle strengthening techniques. Therefore, monolithic ceramiccrowns experience high failure rates range from 4-6% for Dicor molarcrowns (Malament et al. (1999) J. Prosthet. Dent. 81: 23-32) and 3-4%per year for IPS Empress crowns (Fradeani et al. (1997) Int. J.Prothodont. 10: 241-7; Sjogren et al. (1999). Int. J. Prosthodont. 12:122-8). Note: comprehensive clinical reports on the new IPS e.max Presscrowns are not available at this stage.

The current approach to the fracture problem of monolithic crowns is alayer-structure with aesthetic but weak porcelain veneers fused ontostrong but opaque ceramic cores. This involves an increase incrystalline content (from approximately 40 vol % to 99.9 vol %)accompanied by a reduction in glass content. The first successfulstrengthened core ceramic was made of feldspathic glass filled withapproximately 40 vol % alumina particles (McLean et al. (1965). Br.Dent. J. 119: 251-67). The alumina fillers increased the flexuralstrength of the ceramic to approximately 120 MPa with a trade off intranslucency; hence veneering was required. Using McLean's approach, in1983 Coors Biomedical (Golden, Colo.) developed Cerestore all-ceramiccrowns with a ceramic core containing ˜60 vol % of alumina (Sozio et al.(1983). J. Prosthet. Dent. 69: 1982-5). However, following problems withfractured crowns the manufacturer withdrew the system. A similar productfrom the same era, the Hi-Ceram crown (Vita, Bad Säckingen, Germany)with its core material containing about the same amount of alumina asthe Cerestore core, also failed to meet the satisfactory for posteriorrestorations (Bieniek et al. (1994). Schweitz Monatsschr Zahnmed 104:284-9). The Hi-Ceram crown was replaced by In-ceram crown (Vita) in1990. The In-ceram crown had a core that was fabricated by lightlysintering an alumina powder compact and then infiltrating the stillporous alumina matrix with a low viscosity glass. The final corematerial contained approximately 70 vol % of alumina and had a flexuralstrength of approximately 450 MPa (Probster (1992) Int J Prosthodont5(5): 409-14). In 1993, Procera (Nobel Biocare, Göteborg, Sweden)presented the all-ceramic crown concept (Anderson et al. (1993). ActaOdontol Scand 51: 59-64), where the fully dense core material contained99.9 vol % alumina and displayed a flexural strength of 675 MPa. Severalyears later, even stronger Y-TZP ceramic was introduced to dentistry asa core material with a flexural strength over 1200 MPa.

Although documentation regarding the clinical performance of thezirconia core backed crowns is still limited, laboratory in vitro tests(B. Kim et al, (2007) Journal of Dental Research, 86(2): 142-146) andanecdotal clinical reports (Donovan (2005) Journal of Esthetic andRestorative Dentistry 17(3): 141-3) indicate that the zirconia cores arevery fracture resistant. However, a frequent problem is fracture of theporcelain veneer. Despite significant improvements in the performance ofexisting dental ceramics, the structural stability of all-ceramicsystems remains less reliable than metal-ceramic systems (porcelainveneers fused onto metal copings) (Kelly (2004) Dent. Clin. N. Am. 48:513-30). While efforts in improving the structural performance ofall-ceramic crowns have been focused on making monolithic materialsstronger or fabricating stronger cores to support weak, but aestheticporcelain veneers, few innovative approaches have emerged to developmore damage resistant and longer lasting ceramic crowns. This is due inpart to the lack of current knowledge of damage modes that could occurin a ceramic crown under mastication.

Unfortunately, no current materials, including monolithic ceramicsstronger (orthopedic and dental prostheses) or strong cores to supportweak, but aesthetic porcelain veneers (dental prostheses) caneffectively suppress both contact and flexural damages. In addition,veneered strong ceramic dental prostheses have a dense, high puritycrystalline structure at the cementation internal surface that cannot bereadily adhesively bonded to tooth dentin as support. Surface rougheningtreatment such as particle abrasion is commonly used to enhance theceramic-luting agent bond. However, particle abrasion also introducessurface flaws or microcracks that can cause deterioration in thelong-term flexural strength of ceramic prostheses. (Zhang et al. (2004)Journal of Biomedical materials research 71B(2): 381-6; Zhang et al.(2005) Journal of Biomedical materials research 72B: 388-92; Zhang etal. (2006) The International Journal of Prosthodontics 19(5): 442-8).

Recent advances in theoretical and experimental work have shown thatfunctionally graded materials, referred to as FGMs, may provideunprecedented resistance to contact damage (Suresh et al. (2003) U.S.Pat. No. 6,641,893; Suresh et al. (1997) Acta Materialia 45(4): 1307-21;Jitcharoen et al. (1998) Journal of the American Ceramic Society 81(9):2301-8; Suresh et al. (1999) Acta Materialia 47(14): 3915-3926). Suchdamage resistance cannot be realized with conventional homogeneousmaterials. FGMs are made of two materials that are combined so that thesurface of the FGM is composed entirely of material A, and the interioris composed entirely of material B. Additionally, there is a continuouschange in the relative proportions of the two materials from the surfaceto interior. One known FGM is a thick ceramic block, alumina or siliconnitride, infiltrated with a low elastic modulus aluminosilicate glass oroxynitride glass (SiAlYON), respectively, on one surface to produce agraded glass/ceramic (G/C) structure that suppresses contact damage atthe top, occlusal surface (Jitcharoen et al. (1998) Journal of theAmerican Ceramic Society 81(9): 2301-8). However, upon infiltration ofdense ceramics, the glass penetrates the grain boundaries and grainboundary triple junctions, and as a result, the ceramic grains graduallyseparate. This leads to an increase in volume at the surface of gradedstructure and is accompanied by warpage or bending of the specimenswhere the glass-impregnated surface is convex.

Zirconium dioxide (ZrO₂), sometimes known as zirconia, is a whitecrystalline oxide of zirconium. Its most naturally occurring form, witha monoclinic crystalline structure, is the rare mineral, baddeleyite.Pure ZrO₂ has a monoclinic crystal structure at room temperature andtransitions to tetragonal and cubic at increasing temperatures. Thevolume expansion caused by the cubic to tetragonal to monoclinictransformation induces very large stresses, and will cause pure ZrO₂ tocrack upon cooling from high temperatures. Several different oxides areadded to zirconia to stabilize the tetragonal and/or cubic phases:magnesium oxide (MgO), yttrium oxide, (Y₂O₃), calcium oxide (CaO), andcerium oxide (Ce₂O₃), amongst others.

If sufficient quantities of the metastable tetragonal phase zirconia ispresent, then an applied stress, magnified by the stress concentrationat a crack tip, can cause the tetragonal phase to convert to monoclinic,with the associated volume expansion. This phase transformation can thenput the crack into compression, retarding its growth, and enhancing thefracture toughness. This mechanism is known as transformationtoughening, and significantly extends the reliability and lifetime ofproducts made with partially stabilized zirconia. A special case ofzirconia is that of tetragonal zirconia polycrystalline or TZP, which isindicative of polycrystalline zirconia composed of only the metastabletetragonal phase. This material is also used in the manufacture offrameworks for the construction of dental restorations such as crownsand bridges which are then veneered with a dental feldspathic porcelain,as well as femoral heads for the total hip replacement.

SUMMARY OF INVENTION

The present invention takes advantage of the discovery that fractureproblems of ceramic prostheses are minimized by a new generation ofdamage resistant ceramic prostheses utilizing functionally gradedmaterials (FGMs). The present invention represents an improvement overthe G/C structure of the prior art to a graded G/C/G structure byinfiltrating top and bottom ceramic surfaces with glass. The presentinvention features a structure of G/C/G comprising an outer surfaceaesthetic residual glass layer, a graded glass-ceramic layer, and adense interior ceramic.

In a first aspect, the present invention provides a method for preparinga functionally graded glass/ceramic/glass (G/C/G), preferably afunctionally graded glass/zirconia/glass (G/Z/G) sandwich material,comprising: (a) applying a powdered glass-ceramic composition toaccessible surfaces of a presintered zirconia substrate thereby coveringthe zirconia substrate surfaces with a layer of the composition whereinthe coefficient of thermal expansion (CTE) of the glass-ceramic and thecoefficient of thermal expansion (CTE) of the substrate material aresubstantially the same; and (b) infiltrating the glass-ceramiccomposition into the substrate and densifying the substrate by heatingthe substrate. In some embodiments, the heating is performed toapproximately the sintering temperature of the substrate.

In some embodiments, the substrate comprises yttria-tetragonal zirconiapolycrystal (Y-TZP). In other embodiments, the substrate is presinteredat a temperature of from about 900° C. to about 1400° C. In yet otherembodiments, the densifying is performed in one or more firing cycles ata temperature of from about 1200° C. to 1550° C., or 1300° C. to 1500°C., preferably from about 1400° C. to 1450° C. Also, in someembodiments, the powdered glass-ceramic composition is dispersed in anaqueous based solution. It is preferred that the powdered glass-ceramiccomposition comprises one or more oxides selected from the groupconsisting of SiO₂, Al₂O₃, K₂O, Na₂O, BaO, Tb₄O₇, and CaO. Each of theone or more oxides may be present in a weight percent of about 1%, 2%,5%, 10%, 15%, 20%, 25%, 30%, or even 50%. In some embodiments only ofthe oxides is present, while in other embodiments, two, three, four,five, six, seven or even eight of the oxides may be present. Also, inparticularly preferred embodiments, the CTE of the glass and the CTE ofthe zirconia are substantially the same. That is, when the CTEs aresubstantially the same, in some embodiments, the CTE of the glass andthe CTE of the zirconia are within about 50%, 40%, 30%, 25%, 20%, 10%,5%, 2%, 1% or even 0.5% or 0.25% of each other. In especially preferredembodiment, the CTE of the glass is approximately about 10.0 to 11.0, or10.3 in/in/° C., from 0 to 430° C., and the CTE of the zirconia isapproximately about 10.0 to 11.0, or 10.3 in/in/° C., from 0 to 430° C.In some embodiments, the presintered zirconia substrate is presinteredby a microwave technique, and in some embodiments, theglass/zirconia/glass (G/Z/G) sandwich material is densified by amicrowave technique.

In a second aspect, the present invention provides a functionally gradedglass/ceramic/glass composite structure comprising an outer residualglass layer, an underlying graded glass-ceramic layer, and a denseinterior ceramic. In some embodiments, the functionally gradedglass/ceramic/glass composite structure is substantially non-susceptibleto warppage or bending. The functionally graded glass/zirconia/glass(G/Z/G) sandwich material may be produced in some instances by themethod described above as a first aspect of the invention.

In some embodiments, the functionally graded glass/ceramic/glasscomposite structure is composed of an underlying ceramic madesubstantially of yttria-tetragonal zirconia polycrystal (Y-TZP). In someembodiments, the CTE of the glass and the CTE of the zirconia aresubstantially the same. That is, when the CTEs are substantially thesame, in some embodiments, the CTE of the glass and the CTE of thezirconia are within about 50%, 40%, 30%, 25%, 20%, 10%, 5%, 2%, 1% oreven 0.5% or 0.25% of each other. In especially preferred embodiment,the CTE of the glass is approximately about 8.0 to 15.0, 10.0 to 11.0,or 10.3 in/in/° C., from 0 to 430° C., and the CTE of the zirconia isapproximately about 8.0 to 15.0, 10.0 to 11.0, or 10.3 in/in/° C., from0 to 430° C.

In some embodiments, the outer glass layer may be from 5 to 1,000microns thick, sometimes 10 to 750 microns thick, or 20 to 500 micronsthick, or 25 to 250 microns thick, or 30 to 100 microns thick, forinstance. Likewise, in some embodiments, the graded glass-ceramic layermay be from 10 to 500 microns thick, or 20 to 300 microns thick, or 30to 200 microns thick, or 40 to 150 microns thick, or 50 to 125 micronsthick, or 60 to 100 microns thick, for instance.

In a third aspect, the present invention provides a prosthesiscomprising a functionally graded glass-ceramic/ceramic/glass-ceramiccomposite structure or a graded glass-ceramic/ceramic structure. Theprosthesis may be, for instance, an aesthetic and damage-resistantceramic orthopedic prosthesis, orthopedic stems, orthopedic/dentalanchors, orthopedic/dental implants, dental prostheses, and endodonticposts. The structure may comprise an outer residual low modulus glasslayer, an underlying graded glass-ceramic layer, and a dense interiorceramic.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a preferred processing methodfor the fabrication of a flat G/Z/G composite.

FIG. 2 provides optical microscope images showing cross-sectional viewsof polished G/Z/G FGMs (d=1.5 mm) fabricated from Y-TZP platespresintered at (a) 1400° C. for 1 hour and (b) 1100° C. for 1 hour.Glass infiltration and densification were carried out at 1450° C. for 2hours in air. Different thicknesses of surface graded glass-ceramiclayers in the two cases. Inserts in FIG. 2 b are SEM images showingsurface graded layer containing high glass content whereas the interiorconsists of dense Y-TZP. (c) A low magnification SEM image of G/Z/Gfabricated from 1100° C. for 1 hour presintered specimen revealing (fromleft to right): an outer surface residual glass layer (approximately 6μm thick) and a graded glass-zirconia layer. The glass content (black)gradually decreases as proceeding towards the interior.

FIG. 3 shows XRD spectra of (a) homogenous Y-TZP sintered at 1450° C.for 2 hours; (b) Y-TZP presintered at 1100° C. for 1 hour; and (c) G/Z/Gfrabricated from infiltration and densification of presintered (1100° C.for 1 hour) at 1450° C. for 2 hours. T: tetragonal zirconia phase, andG: amorphous glass phase. No secondary crystalline phase exists in G/Z/GFGMs. Spectra acquired using CuKα radiation with a scan rate of 1°/minand a step size of 0.2°.

FIG. 4 is a bar chart depicting critical loads for internal surfaceradial cracking of ceramic plates (d=1.5 or 0.4 mm) on polycarbonatesubstrates. Ceramic plates are G/Z/G fabricated from glass infiltrationof 1400° C. presintered Y-TZP and the bulk Y-TZP. Note the advantage ofG/Z/G over Y-TZP is more pronounced for thinner samples.

FIG. 5 is a digital photograph depicting (a) a G/Z/G plate (1.5 mmthick) fabricated from glass infiltration of 1400° C. presintered Y-TZPusing a glass-ceramic composition 1. For comparison, a monolithic glassceramic (b) Empress II, porcelain veneered zirconias (c) Lava and (d)Cercon of 1.5 mm thickness are also shown. Also, a one cent coin isshown (e) for size reference.

FIG. 6 is a schematic diagram illustrating a ceramic liner and a ceramicfemoral head with (a) and (b) both surfaces graded or (c) and (d) onlyone surface graded for an orthopedic prosthesis. (e) Ceramic dentalprosthesis (for both monolithic or core structures) with gradedstructures at surfaces subject to wear, contact, and impact.

FIG. 7 is a schematic illustration of crack geometry for cyclic loadingof (a) monolith ceramic coatings and (b) veneered ceramic layers oncompliant substrates with sphere of radius r at load P in water.Near-contact surface damage modes: outer cone (O); inner cone (I);median crack (M). Far-field internal surface radial crack (R).

DETAILED DESCRIPTION OF THE INVENTION

An FGM structure where a thick ceramic block, alumina or siliconnitride, is infiltrated with a low elastic modulus aluminosilicate glassor oxynitride glass, respectively, on one surface to produce a gradedglass/ceramic (G/C) structure that suppresses contact damage at the top,occlusal surface is known in the art. (Jitcharoen et al. (1998) Journalof the American Ceramic Society 81(9): 2301-8) The present inventionprovides a graded G/C/G structure by infiltrating top and bottom ceramicsurfaces with glass. The G/C/G structure suppresses both occlusalsurface contact damage and cementation internal surface flexural damage.In addition, the unique structure of G/C/G, an outer surface aestheticresidual glass layer, a graded glass-ceramic layer, and a dense interiorceramic (FIG. 2), allows optimizing optical and cementation properties.

The FGM structure of the present invention having a low modulus glassceramic at both the top and the bottom surfaces, sandwiching a highmodulus, strong ceramic interior, improves resistance to both contactand flexural damage. In addition, the FGM structure of the presentinvention together with outer surface residual glass layers may be usedto enhance the aesthetics and cementation behavior of polycrystallinedental ceramic cores, including the exceptionally strong class ofzirconia ceramics. Moreover, it is possible to optimize the thickness ofthe surface graded layer and residual glass layer thereby providing thebest combination of aesthetics, resistance to contact damage andflexural fracture for G/C/G FGMs.

Glass-ceramic powders are taught in U.S. Provisional Patent ApplicationSer. No. 60/860,165, the disclosure of which is herein incorporated byreference in its entirety. The present invention provides G/Z/Gstructures having a thickness useful for dental applications. In manyembodiments, the glass-ceramic powders used for infiltrating G/Z/Gcontain one or more of, but are not restricted to, the following mainoxides (i.e. at 1.0 weight percent or more): SiO₂, Al₂O₃, K₂O, Na₂O,BaO, Tb₄O₇, and CaO. The composition of the infiltrating glass-ceramiccan vary, as long as its CTE is similar or preferably approximately thesame as that of the Y-TZP in a temperature range between theglass-ceramic transition temperature (T_(g)) and room temperature andthe final product has an aesthetic appearance. Silica based glass has apoor permeability in dense Y-TZP even at approximately 1450° C., whichis similar to the sintering temperature of this material. In addition,post-sintering glass infiltration at this temperature may result ingrain growth and/or destabilizing of the tetragonal phase, which areknown to be deleterious for hydrothermal stability of Y-TZP in the body(Chevalier, et al. (2004) Biomaterials 25: 5539-45; Chevalier (2006)Biomaterials 27: 534-43). Therefore, it is preferred to infiltrate thepresintered Y-TZP and to combine infiltration and densification in oneprocess. By doing so, the thickness of the graded glass-ceramic layermay be controlled by the porosity of presintered bodies. Further,combining infiltration and densification in one process can avoid graingrowth and destabilizing of the tetragonal phase.

The G/C/G system of the present invention suppresses both occlusalsurface contact damage and cementation internal surface flexural damage.The G/C/G system of the present invention substantially overcomes thewarpage or bending problems associated with the G/C systems of the priorart. The unique structure of the present invention G/C/G, which providesan outer surface aesthetic residual glass layer, a graded glass-ceramiclayer, and a dense interior ceramic provides the advantage that opticaland the cementation properties may be optimized. FGMs with low modulusglass ceramics at both top and bottom surfaces, sandwiching a highmodulus, strong ceramic interior, improve the resistance to both contactand flexural damage. Such graded structures together with the outersurface residual glass layers may be utilized to enhance the aestheticsand cementation behavior of polycrystalline dental ceramic cores.

In a copending provisional application, U.S. Provisional PatentApplication Ser. No. 60/860,165, the disclosure of which is hereinincorporated by reference, G/C/G structures are disclosed based upon asandwiched layer of alumina. The present invention is based in part uponthe unexpected discovery that FGMs having surprisingly superiorproperties are produced when the sandwiched layer comprises theexceptionally strong class of zirconia ceramics. It has been shown thatcontinuously graded G/C composites, without significant internalstresses, may be produced by infiltrating glass into a dense ceramicsurface where the two constituents G and C possess similar coefficientsof thermal expansion (CTEs) and Poisson's ratio. Zirconia, morespecifically yttria-tetragonal zirconia polycrystal (Y-TZP), is farsuperior to alumina in terms of mechanical properties, and the G/Z/Gsystem used in the present invention provides robust, aesthetic, thinall-ceramic prostheses for less invasive posterior applications.However, the permeability of silica based glass in dense Y-TZP is pooreven at temperatures near its sintering temperature. In addition,post-sintering glass infiltration at temperatures near the sinteringtemperature results in grain growth and/or destabilization of thetetragonal phase, which in turn deteriorates the hydrothermal stabilityof Y-TZP (Chevalier, et al. (2004) Biomaterials 25: 5539-45).

The present invention provides a G/Z/G FGM produced by infiltrating thesurfaces of presintered Y-TZP with a powdered glass-ceramic slurry whichhas a similar CTE and Poisson's ratio to those of Y-TZP, and bycombining infiltration and densification in one stage. By the term“presintered” is meant that the powdered composition of the substratehas been subjected to an elevated temperature/time heating schedule, butyet below that which would effect full densification of the compound. Inthis manner, complications arising from the post-sinteringinfiltration/heat-treatment can be avoided and the various graded layerthicknesses can be produced by controlling the porosity of presinteredbodies using different presintering temperatures. Further, theaesthetics of FGMs is governed by the thickness of the surface residualglasses and the microstructure of graded layers. Therefore, an optimalthickness of the graded layer and surface residual glass layer thatresults in the best combination of aesthetics, resistance to contactdamage and flexural fatigue is provided. The new G/Z/G composites offerbetter resistance to flexure-induced damage, better aesthetics, betterveneering and cementation properties, and better resistance tohydrothermal degradation over the existing commercial Y-TZP cores.

The appearance of G/Z/G infiltrated on partially dense bodiespresintered at 1400° C. is shown in FIG. 6, along with commercialmonolithic glass ceramic Empress II (1.5 mm thick), veneered Lavazirconia (1 mm porcelain and 0.5 mm zirconia) and Cercon zirconia (1 mmporcelain and 0.5 mm zirconia). For better aesthetics, a thin veneer maybe applied to the outer surface of G/Z/G. This thin veneer contains thecontact damage, provides aesthetics, prevents unwanted wear of opposingnatural dentition, and allows for adjustment on the occlusal surface.Any occlusal-surface contact damage will be confined within the thinveneer layer, because cracks are unlikely to propagate from a lowmodulus, low toughness porcelain to a high modulus, high toughness Y-TZP(Kim et al. (2006) J Biomed Mater Res B Appl Biomater 79(1): 58-65). Theunique structure of G/Z/G, including a surface aesthetic residual glasslayer, a graded glass-zirconia layer, and a dense interior Y-TZP layerallows for a thin veneer. The thickness of the surface aesthetic glasslayer varies from 5 to 50 μm, depending on the fabrication conditions.Although the glass-zirconia graded layer has limited translucency due toits high crystalline content, it provides a gradual transition intranslucency from the highly translucent porcelain veneer and surfaceresidual glass layer to the opaque Y-TZP interior, which allows for theoptical depth necessary in creating the right aesthetic outcome.Alternatively, color stains can be applied to the surface of the outerresidual glass layer of G/Z/G using a powdered glass slurry that hassimilar composition to the infiltrated glass. This staining techniquehas been used on the Empress system to improve the aesthetic outcome ofa single colored pressed block of glass-ceramic and is well establishedin aesthetic dentistry.

The present invention provides a uniform graded layer on both top andbottom surfaces of Y-TZP plates using glass infiltration technique. Thistechnique can be readily used to fabricate graded structures on surfacesof orthopedic and dental prostheses with complex geometry (FIG. 6).

An objective of the invention is to develop robust, aesthetic, thinceramic crowns and bridges for less invasive posterior restorativeprotocols. A G/Z/G material offers better resistance to flexure-induceddamage, better aesthetics, and better veneering and cementationproperties than bulk Y-TZP. A G/Z/G material eliminates sharp boundariesin veneered Y-TZP prostheses, which may lead to delamination of theporcelain veneer due to the dissimilar physical and mechanicalproperties of porcelain and Y-TZP. The residual glass and the glasscontaining graded layer on the internal side of G/Z/G offers robustadhesive bonding using, for example, etching-silane techniques ratherthan a traditional grit-blasting procedure. A traditional grit-blastingprocedure may induce damage on the internal side of a dentalrestoration, resulting in strength degradation (Zhang et al. (2004)Journal of Biomedical materials research 71B(2): 381-6; Zhang, Y., B. R.Lawn, et al. (2006) The International Journal of Prosthodontics 19(5):442-8). With an increase in resistance to flexural damage, the absenceof grit-blasting damage, and the aid of adhesive cementation, theoverall strength of the G/Z/G restoration is much higher than currentveneered zirconia restorations. In addition, the current fixed partialdentures (FPDs) with Y-TZP framework often fracture from the lowerportions of the connectors, leading to chipping or delamination of theporcelain veneer. A G/Z/G structure provides improved aesthetics, whichallows for a FPD design without porcelain veneering in the lowerportions of the connectors, improving the flexural damage resistance ofPFDs. Finally, the residual glass at the G/Z/G surfaces acts as anencapsulation layer that may impede water absorption and preventhydrothermal degradation of interior Y-TZP (Piascik et al. (2006) J.Vac. Sci. Technol. A 24(4) 1091-5). This can lead to the development ofstrong yet aesthetic ceramics for posterior inlays, onlays, crowns andbridges.

Fracture Mechanics Analysis

Damage in brittle ceramics loaded with a cylindrical or curved indenterwas explored in detail in the late 1800s by Hertz who describedcharacteristic fracture patterns called Hertzian or classical conecracks (Hertz (1882) J. Reine und Angewandte Mathematik 92:156-171;Hertz (1896) Hertz's Miscellaneous Papers. London, Chs. 5,6: Macmillan).Intense research concerning damage modes in brittle coatings oncompliant substrates loaded on the top surface, emulating ceramic crownson dentin, began in the late 1980s. Most of the tests were done undersingle-cycle loading with a hard sphere indenter. Several damage modes,summarized in FIG. 7 were identified and analyzed. They can be dividedinto two categories: top-surface (occlusal-surface) damages fromnear-contact stresses, and bottom-surface (cementation internal surface)damage from far-field flexural stresses.

Near-contact occlusal-surface fracture modes in brittle materials,including outer cone cracks and median cracks, formed by precursorquasiplastic deformation. Outer cone cracks (O, FIG. 7) initiate justoutside the indenter contact area where the maximum tensile stress ofHertizan stress field occurs. Quasiplastic deformation forms beneath theindenter, producing grain boundary microcracks which coalesce and evolveinto occlusal-surface median cracks (M, FIG. 7). For brittle dentalceramics like porcelain and alumina, classical cone cracks dominate.

Far-field cementation internal surface radial fractures (R, FIG. 7)result from tensile stresses generated during loading. Radial cracks areoriented normal to the plate surface and are susceptible to any flexuraltensile stresses generated during function. Therefore, once initiated,they propagate sideward and upward, ultimately leading to fracture(Kelly (1999) The Journal of Prosthetic Dentistry 81(6): 652-61). Indental crowns, radial cracks are clinically evidenced as bulk fracturewhich is believed to constitute the primary mode of failure ofall-ceramic crowns. The load to initiate these internal surface radialcracks (P_(r)) depends strongly on thickness and elastic modulus of theceramic and substrate and is given by P_(r)=Bσ_(c)d²/(log E_(c)/E_(s)),where B is a constant, σ_(c) is the flexural strength of the material, dis the ceramic layer thickness, E_(c) is the elastic modulus of theceramic, and E_(s) is the elastic modulus of the supporting substrate.

Extensive testing of porcelains, aluminas, zirconias, and glass ceramicson compliant structures have provided the data that has ultimately leadto fundamental relationships concerning loads to damage initiation forouter, median, and radial cracks for this broad array of ceramic layerson compliant structure for clinically relevant thickness undersingle-cycle loading. While there is competition for all outer, median,and radial modes to develop, in general radial cracks are likely toinitiate first in thin sections (<0.8-1.0 mm), outer and median todevelop first in thicker sections. The next goal is to develop amaterial with improved resistance to all these damage modes and wearwhile not increasing the hardness, elastic modulus, and fracturetoughness of the surface of the prostheses, to avoid excessive wear ofthe opposing tooth or crown.

Damage Resistance of FGMs

The theoretical framework concerning frictionless normal indentation ofelastically graded materials from a point load or from differentindenter geometries has been developed by Giannakopoulos and Suresh.Explicit analytical expressions have been developed to relate theindentation load P to the penetration depth h, the contact radius a, andcontact pressure p₀, for a Young's modulus E which varies with depth zbeneath the indented surface. Theory predicts that when the elasticmodulus increases with depth, the stress fields for the power-law caseare focused more in the interior than for the corresponding exponentialcase. Experimental studies showed when glasses infiltrate into a denseceramic surface, the Young's modulus variation from surface to interioris best described by the power-law relation E=E₀z^(k), where E₀ is thereference Young's modulus at the surface and k is a dimensionlessconstant (Jitcharoen et al. (1998) Journal of the American CeramicSociety 81(9): 2301-8). Such elastic variation effectively transfers themaximum contact stresses into interior upon occlusion, resulting in muchimproved resistance to quasiplastic deformation and brittle fracture ator in the vicinity of the occlusal surface.

When a ceramic plate mounted onto a less stiff substrate (tooth-dentin)is subjected to loading from the top surface with a sphere indenter, thebottom surface of the ceramic plate experiences a maximum tensile stresswhich can result in bottom surface R cracking (FIG. 7). Finite ElementAnalysis (FEA) of FGMs with an increasing elastic modulus from thebottom surface to interior shows that the maximum tensile stress couldbe lowered by 20% compared to its bulk ceramic counterpart, even if thegraded layer at the ceramic bottom surface is only 200 μm thick (Huanget al. (2007) J Mater Sci Mater Med 18(1):57-64). This is because theFGM at the bottom surface spreads the maximum tensile stresses from thesurface into the interior. Therefore, if both top and bottom ceramicsurfaces are graded, the damage modes shown in FIG. 7 can all besuppressed.

Ceramic crowns are vulnerable to near-contact and far-field flexureinduced fracture from concentrated loading. Their vulnerability isexacerbated by damage, fatigue loading, and moisture. Although there hasbeen immense amount of study concerning the fracture of ceramic crowns,the bulk of the work reported in the literature has focused on simpleflexural strength tests under monotonic loading (Guazzato et al. (2004)Dental Materials 20: 449-456; Guazzato et al. (2004) Biomaterials 25:5045-5052) or residual strength measurement following cyclic fatigueusing load-to-fracture crushing test (Jung et al. (2000) Journal ofDental Research 79(2): 722-31; Stappert et al. (2005) Journal ofProsthetic Dentistry 94(2): 132-139). These tests may not accuratelypredict the lifetime of real ceramic crowns, because most dentalceramics are susceptible to moisture assisted slow crack growth, whichcan result in a reduction in strength by 50% or more over a year or so(Zhang et al. (2004) Journal of Biomedical Materials Research 69B:166-72). Also, ceramics are susceptible to cumulative mechanical damageduring contact loading. It is important to systematically analyzefracture modes and damage evolution in ceramic layers inclinically-relevant testing—namely cyclic loading beneath a sphericalindenter in a wet environment. A new damage mode, inner cone fracture,has been identified (FIG. 7). It is now well-appreciated that crackinitiation and evolution is complex. Competing failure modes may developon different surfaces, at different stages, and may interact dependingon ceramic properties, layer thicknesses, and loading conditions (Zhanget al. (2005) Journal of Materials Research 20(8): 2021-9).

Glass/Zirconia/Glass Structure

A G/Z/G structure offers better resistance to flexure induced damage(FIG. 4), better aesthetics, and better veneering and cementationproperties over bulk Y-TZP. G/Z/G eliminates sharp interfaces inveneered Y-TZP prostheses, which may ordinarily lead to delamination ofthe porcelain veneer due to the dissimilar physical and mechanicalproperties of porcelain and Y-TZP (Sundh et al. (2004) Journal of OralRehabilitation 31(7): 682-8; Vult von Steyern et al. (2006) Journal ofOral Rehabilitation 33(9): 682-9; Wood et al. (2006) J. Prosthet. Dent.95(1): 33-41). The residual glass and the glass containing graded layeron the internal side of G/Z/G offer great potential for adhesive bondingusing etching-silane techniques rather than the current grit-blastingprocedure, which induces damage on the internal side of a dentalrestoration, resulting in strength degradation (Zhang et al. (2004)Journal of Biomedical Materials Research 69B: 166-72). With an increasein resistance to flexural damage, the absence of grit-blasting damage,and the aid of adhesive cementation, the overall strength of the G/Z/Grestoration is much higher than current veneered zirconia restorations.In addition, the current fixed partial dentures (FPDs) with Y-TZPframework often fracture from the lower portions of the connectors,leading to chipping or delamination of the porcelain veneer. G/Z/G hasimproved aesthetics, which allows for a FPD design without porcelainveneering in the lower portions of the connectors, improving theflexural damage resistance of PFDs. Finally, the residual glass at theG/Z/G surfaces acts as an encapsulation layer that may impede waterabsorption and prevent hydrothermal degradation of interior Y-TZP(Piascik et al. (2006) J. Vac. Sci. Technol. A 24(4) 1091-5). Thisallows strong yet aesthetic ceramics for posterior inlays, onlays,crowns, and bridges.

The invention is further illustrated by the following Examples which areintended to be illustrative of the invention. Those of skill in the artmay vary many experimental parameters within the scope of the appendedclaims.

Example 1

Green compacts were formed from a yttria-stabilized zirconia powder,5.18 wt % Y₂O₃, 0.25 wt % Al₂O₃, and a mean particle size of diameterapproximately 28 nm with a specific surface area of 16 m²/g (TZ-3Y-Egrade, Tosoh, Tokyo, Japan) using a cold isostatic press at 172 MPa (25kpsi). The green compacts were presintered at temperatures between 1100and 1400° C. for 1 hour in air. Infiltration and densification werecarried out at 1450° C. for 2 hours using a custom developedglass-ceramic powder of the type described above. A heating and coolingrate of 800° C./hour was employed. Two glass-ceramic compositions wereprepared for infiltration of Y-TZP. The glass-ceramic powder composition1 consisted of the following main oxides (i.e. at 1.0 weight percent ormore): SiO₂ (67.25 wt %), Al₂O₃ (10.83 wt %), K₂O (9.22 wt %), Na₂O(6.61 wt %), CaO (2.68 wy %), Tb₄O₇ (1.84 wt %), BaO (1.02 wt %), and asmall amount of MgO. The glass-ceramic powder composition 2 consisted ofSiO₂ (67.42 wt %), Al₂O₃ (11.42 wt %), K₂O (9.12 wt %), Na₂O (6.29 wt%), CaO (2.74 wt %), Tb₄O₇ (1.51 wt %), BaO (1.19 wt %) and a smallamount of Ce₂O₃.

Optical microscope images of G/Z/G fabricated from presintered bodiesusing glass-ceramic composition 1 are shown in FIG. 2. Graded layers atboth surfaces of G/Z/G plates are approximately 60 and 150 μm thick for1400 and 1100° C. presintered specimens respectively (FIGS. 2 a and b).Higher magnification SEM (inserts of FIG. 2 b) revealed that the gradedlayer consists of a high glass content (approximately 40 vol. %) whereasthe interior comprises dense Y-TZP. A thin aesthetic residual glasslayer is observed on the surfaces of G/Z/G FGMs (FIG. 2 c); typicallybeing <10 μm for 1100° C. presintered and <30 μm for 1400° C.presintered specimens respectively.

Example 2

A standard three-point bending test with a span of 20 mm was used tofracture rectangular bar specimens at a crosshead speed of 1 mm/min on acomputer-controlled universal testing machine (model 5566, InstronCorp., Canton. MA). Flexural strength, σ, was determined using theequation below for homogeneous zirconia and for the two G/Z/Gcompositions fabricated from infiltrating 1400° C. for 1 hourpresintered Y-TZP with in-house prepared glass-ceramic powders(composition 1 or 2) at 1450° C. for 2 hours:

σ=3Pl/2wb ²

where P is the breaking load; l is the test span; and w and b are thespecimen width and thickness, respectively. Flexural strengths of thetwo G/Z/G compositions and the homogeneous zirconia control are reportedin Table 1. Data are presented in the form of mean and standarddeviation (mean±SD) for a specimen size n=6. As can be seen, flexuralstrengths for G/Z/G infiltrated with glass-ceramic composition 1 and 2were approximately 43% and 47%, respectively, higher than those forhomogenous Y-TZP specimens. 1-sample t-test showed that it was highlyunlikely (p<0.001) that a sample as strong as G/Z/G could have beensampled from the population of homogeneous Y-TZP.

TABLE 1 Flexural strength data of the two G/Z/G compositions fabricatedfrom infiltrating 1400° C. for 1 hr presintered Y-TZP with in-houseprepared glass-ceramic powders (composition 1 or 2) at 1450° C. for 2 hrand their homogeneous Y-TZP counterpart. Flexural strength, σ, MPaSpecimens (mean ± SD) G/Z/G (infiltrated with glass-ceramiccomposition 1) 1443.8 ± 252.2 G/Z/G (infiltrated with glass-ceramiccomposition 2) 1485.8 ± 186.5 Homogeneous Y-TZP 1012.7 ± 158.5

Example 3

Ceramic plates were epoxy bonded to polycarbonate bases and loaded onthe top surface, emulating ceramic dental crowns on tooth dentin supportsubjected to occlusion or ceramic liner on polyethylene backingsubjected articulation in total hip replacement. Critical loads for theonset of ceramic bottom surface radial cracking (an indication offlexural strength of the ceramic plates) were measured for homogeneouszirconia and for G/Z/G (d=1.5 or 0.4 mm) fabricated from infiltrating1400° C. for 1 hour presintered T-YZP with an in-house preparedglass-ceramic powder (composition 1) at 1450° C. for 2 hours. Sixspecimens (n=6) were fabricated from two different batches for eachG/Z/G thickness: 20×20×1.5 mm³ or 20×20×0.4 mm³. Variations in criticalloads between specimens fabricated from the two different batches weresimilar to those prepared from the same batch, being typicallyapproximately 10%. As shown in FIG. 4, critical loads for G/Z/Ginfiltrated at 1400° C. for 1 hour presintered Y-TZP were approximately30% higher than those for homogenous Y-TZP for 1.5 mm specimens, whilecritical loads for G/Z/G fabricated at the same condition was almosttwice that of those for bulk Y-TZP when the specimen thickness wasreduced to 0.4 mm. Again, 1-sample t-test showed a significant omnibustest results (i.e. p<0.001) for both thicknesses.

FIG. 3 provides XRD spectra of (a) homogenous Y-TZP sintered at 1450° C.for 2 hours; (b) Y-TZP presintered at 1100° C. for 1 hour; and (c) G/Z/Gfrabricated from infiltration and densification of presintered (1100° C.for 1 hour) at 1450° C. for 2 hour. T: tetragonal zirconia phase, and G:amorphous glass phase. Note that no secondary crystalline phase existsin G/Z/G FGMs. Spectra were acquired using CuKα radiation with a scanrate of 1°/min and a step size of 0.2°.

Example 4

Using a glass-ceramic powder, initial infiltration conditions tofabricate G/Z/G structures in the thickness necessary for dentalapplications were determined. Preliminary data show that silica basedglass has a limited permeability in dense Y-TZP even at approximately1450° C., which is similar to the sintering temperature of thismaterial. In addition, post-sintering glass infiltration at thistemperature could result in grain growth and/or destabilizing of thetetragonal phase, which are known to be deleterious for hydrothermalstability of Y-TZP in the body (Chevalier (2006) Biomaterials 27:534-43; Chevalier et al. (1999) Journal of the American Ceramic Society82(8): 2150-4). Therefore, it is preferred to infiltrate the presinteredY-TZP and to combine infiltration and densification in one process.Thereby, the thickness of the graded glass-zirconia layer can becontrolled by the porosity of presintered bodies, and combininginfiltration and densification in one process can avoid grain growth anddestabilizing of the tetragonal phase.

Green compacts were formed from a yttria-stabilized zirconia powder,5.18 wt % Y₂O₃, 0.25 wt % Al₂O₃, and a mean particle size of diameterapproximately 28 nm with a specific surface area of 16 m²/g (TZ-3Y-Egrade, Tosoh, Tokyo, Japan) using a cold isostatic press at 172 MPa. Thegreen compacts were presintered at 1100 or 1400° C. for 1 hr in air.Infiltration and densification were carried out at 1450° C. for 2 hoursusing a custom developed glass-ceramic powder (FIG. 1).

FIG. 1 is a schematic diagram illustrating the processing method for thefabrication of flat G/Z/G composite: applying a powdered glass-ceramicslurry at the top and bottom surfaces of presintered Y-TZP (left), andsintering at 1450° C. for 2 hours to form a G/Z/G composite. The G/Z/Gstructure consists of an outer surface aesthetic residual glass layer, agraded glass-zirconia layer, and a dense interior Y-TZP.

Optical microscope images of G/Z/G fabricated from presintered bodiesare shown in FIG. 2. Graded layers at both surfaces of G/Z/G plates areapproximately 60 and 150 μm thick for 1400 and 1100° C. presinteredspecimens, respectively (FIGS. 2 a and 2 b). Higher magnification SEM(inserts of FIG. 2 b) reveals that the graded layer consists of a highglass content (approximately 40 vol. %) whereas the interior comprisesdense Y-TZP. A thin aesthetic residual glass layer is observed on thesurfaces of G/Z/G FGMs (FIG. 2 c) It is typically less than 10 μm for1100° C. presintered and less than 50 μm for 1400° C. presinteredspecimens respectively.

One concern for G/Z/G FGMs is that the crystallization of glass, both insurface residual glass layer and in the graded layer, upon cooling,could modify the CTE and impair the aesthetics of G/Z/G. For thisreason, a glass composition which exhibits excellent resistance tocrystallization upon cooling was formulated. X-ray diffraction (XRD)analysis of G/Z/G FGMs revealed a small amount of glass phase in thesurface residual glass and graded glass-ceramic layers and there is nodetectable secondary crystalline phase present in addition to themetastable tetragonal phase, at least within the detection limit of XRD(i.e. approximately 3 vol. %) (FIG. 3 c). XRD spectrum of a sinteredY-TZP (1450° C. for 2 h) is shown in FIG. 3 a. In addition, nomonoclinic phase is observed in either presintered (FIG. 3 b) orinfiltrated Y-TZP specimens (FIG. 3 c).

Critical loads were measured for polished bulk Y-TZP and G/Z/G plates(d=1.5 or 0.4 mm) fabricated from infiltrating presintered Y-TZP (1400°C. for 1 hour) with an in-house prepared glass-ceramic powder(composition 1) at 1450° C. for 2 hours. Six specimens (n=6) werefabricated from two different batches for each thickness (d=1.5 or 0.4mm). A ˜10% variation in critical load was observed between thespecimens fabricated from the two different batches. As shown in FIG. 4,for 1.5 mm thick specimens, critical loads (mean±SD) for G/Z/G are ˜30%higher than those for bulk-Y-TZP. However, for 0.4 mm thick specimens,critical loads for G/Z/G are almost twice as high as those for bulkY-TZP, suggesting that the impact of graded structure on the flexuraldamage resistance could be more significant for thin (d<0.5 mm) ceramicprostheses. Again, 1-sample t-test shows a significant omnibus testresults (i.e. p<0.001) for both thickness.

The appearance of G/Z/G infiltrated on partially dense bodiespresintered at 1400° C. is shown in FIG. 5, along with commercialmonolithic glass ceramic Empress II (1.5 mm thick), veneered Lavazirconia (1 mm porcelain and 0.5 mm zirconia) and Cercon zirconia (1 mmporcelain and 0.5 mm zirconia). For better aesthetics, we propose toapply a thin porcelain veneer (approximately 0.3 mm thick) to the outersurface of G/Z/G. This thin veneer could contain the contact damage,provide aesthetics, prevent unwanted wear of opposing natural dentition,and allow for adjustment on the occlusal surface. Any occlusal-surfacecontact damage will be confined within the thin veneer layer, becausecracks are unlikely to propagate from a low modulus, low toughnessporcelain to a high modulus, high toughness Y-TZP (Kim et al. (2006). JBiomed Mater Res B Appl Biomater 79(1): 58-65), and the contact damagecontainment predictions will be examined. The thin veneer concept issupported by the unique structure of G/Z/G, which includes a surfaceaesthetic residual glass layer, a graded glass-zirconia layer, and aninterior Y-TZP layer (FIG. 2). Although the glass-zirconia graded layerhas limited translucency due to its high crystalline content, itprovides a gradual transition in translucency from the highlytranslucent porcelain veneer and surface residual glass layer to theopaque Y-TZP interior, which allows for the optical depth necessary increating a good aesthetic outcome. Alternatively, color stains can beapplied to the surface of the outer residual glass layer of G/Z/G usinga powdered glass-ceramic slurry that has similar composition to theinfiltrated glass. This staining technique has been used on the Empresssystem to improve the aesthetic outcome of a single colored pressedblock of glass-ceramic and is well established in aesthetic dentistry.

Example 5

Bilayer specimen fabrication. Table 2 summarizes the materials used.Bilayer specimens of G/Z/G layers on polycarbonate substrates arefabricated. The G/Z/G is based on infiltrating an in-house developedglass-ceramic into the surfaces of presintered Y-TZP (fabricated from afine-grain Y-TZP powder, TZ-3Y-E, Tosoh, Tokyo, Japan) at the sinteringtemperature 1450° C. for 2 hours in air, combining infiltration anddensification in one process. A structure-damage resistance relation isestablished under cyclic loading for this system. Polycarbonate isselected as a support material for the FGMs because it is compliant andcan be considered to represent dentin or bone (though its elasticmodulus is slightly lower than that of either) and is transparent,permitting direct observation of fractures evolving from or propagatingto the bottom surface of the G/Z/G layer. Specimens are loaded using a3.18 mm radius glass or WC sphere.

TABLE 2 Material properties and sources. Thermal Young's expansionmodulus coefficient Poisson's Brand Name Material (GPa) (° C.⁻¹) ratioand Source Glass-ceramic 70 10.3 × 10⁻⁶ 0.26 In-house composition Y-TZP210 10.4 × 10⁻⁶ 0.30 TZ-3Y-E, Tosoh Epoxy resin 3.4 — 0.33 HarcosChemical Resin cement 3.1 Rely x ARC, 3M Polycarbonate 2.3 — 0.33 Hyzod,AIN Plastics Composite 18 Z100, 3M WC 614 — 0.30 r = 3.18 mm, J & LIndustrial Supply

Flat Y-TZP green compacts, fabricated from a fine-grainyttria-stabilized zirconia powder (TZ-3Y-E, Tosoh), are presintered attemperatures between 900 and 1400° C., creating zirconia plates withvarious porosities. The top and bottom surfaces of presintered Y-TZP arecoated with a powdered glass-ceramic slurry which has a similar CTE andPoisson's ratio to those of Y-TZP (Table 2). Glass infiltration anddensification is carried out simultaneously at 1450° C. for 2 hoursinside a high temperature box air furnace (ST-1700C-6612, Sentro TechCorp, Berea, Ohio). A heating and cooling rate of 800° C./hour isemployed. This minimizes grain growth and/or destabilizing of thetetragonal phase associated with the post-sintering heat treatment. Bothgrain growth and destabilizing of the tetragonal phase are deleteriousfor long-term hydrothermal stability of Y-TZP in biomedicalapplications. G/Z/G specimens with two final dimensions are fabricated:20×20×1.2 mm³ or 20×20×0.4 mm³. In addition, by manipulating theporosity of the presintered Y-TZP body, the glass penetration depth maybe controlled, creating G/Z/G structures with various thicknesses of thegraded glass-zirconia layers (FIG. 2). Three groups of specimens withdifferent graded glass-zirconia layer thicknesses are fabricated for aG/Z/G of 0.4 mm total thickness, and six groups with different gradedlayer thicknesses for a G/Z/G of 1.2 mm total thickness (Table 3). Anequal thickness of the graded layer at the top and bottom surfaces foreach specimen is maintained to prevent warpage. The effect of gradedlayer thickness on damage resistance for G/Z/G with differentthicknesses may be elaborated by comparing specimen groups G-Tn1, G-Tn2,G-Tn3 with G-Tk1, G-Tk2, G-Tk3. The effect of relative ratio of thegraded layer and the total specimen thickness on the damage resistancemay be examined by comparing specimen groups G-Tn1, G-Tn2, G-Tn3 withG-Tk3, G-Tk4, G-Tk5. Tn and Tk represent thin (0.4 mm) and thick (1.2mm) specimens respectively.

TABLE 3 Design parameters for G/Z/G with various thicknesses of gradedlayers at top and bottom surfaces. Tn and Tk represent thin (0.4 mm) andthick (1.2 mm) specimens respectively. Total G/Z/G thickness (mm) 0.41.2 Graded layer thickness (mm) G- G- G- G- G- G- G- G- G- Tn1 Tn2 Tn3Tk1 Tk2 Tk3 Tk4 Tk5 Tk6 Top 0.05 0.10 0.15 0.05 0.10 0.15 0.30 0.45 0.55surface Bottom 0.05 0.10 0.15 0.05 0.10 0.15 0.30 0.45 0.55 surface

The excess glass may be ground away from the G/Z/G surfaces and theplates will be epoxy bonded (Harcos Chemicals, Bellesville, N.J.) to a12.5 mm thick polycarbonate substrate (Hyzod, AlN Plastics, Norfolk,Va.) for single-cycle-load screen test using a spherical tungstencarbide indenter (r=3.18 mm) to determine the flexure induced damageresistance of the G/Z/G cores. Only the two strongest groups (requiringhigher loads to fracture) from the single-cycle-load screen results foreach specimen thickness 1.2 or 0.4 mm is chosen for cyclic fatigue teststo construct the design maps. Specimens with polished internal surfacesbut with residual aesthetic glass on the occlusal surface are epoxybonded to polycarbonate substrates for fatigue tests using a sphericalglass indenter (r=3.18 mm) in water. The stronger group out of the twogroups subjected to cyclic fatigue test is selected to determine theeffect of surface treatment on the damage resistance of G/Z/G. Specimensof 1.2 mm thick with abraded or etched internal surfaces are epoxybonded to polycarbonate substrates for fatigue tests using a glassindenter in water. As-polished surfaces yield the intrinsic strength ofFGMs but may not represent real-life conditions. Laboratory and clinicalpractices (e.g., particle abrasion or etching of the internal surface ofall-ceramic crowns and bridges) may damage the surface. To mimic this,the ceramic bottom surface is abraded with 50 μm alumina particles for 5seconds from a standoff distance of 10 mm using 276 KPa compressed airpressure or etched with HF solution (9.5%) for 90 seconds. Finally, thestronger group out of the two groups subjected to cyclic fatigue testsis cemented (Rely X ARC, 3M/ESPE, St. Paul, Minn.) to a 5 mm thick Z100substrate (3M/ESPE, St. Paul, Minn.) for step-stress fatigue tests usinga glass indenter in water. The internal surface of G/Z/G is abraded oretched, based on the treatment that will yield a better damageresistance from the preceding tests. To simulate the aesthetic veneer, athin porcelain veneer may be placed on the occlusal surface of the G/Z/Gstructure (1.2 mm G/Z/G and 0.3 mm porcelain). Since extensive data onflat bilayer specimens have been generated, some additional flatspecimens, namely Lava zirconia plate on polycarbonate and porcelainveneered Lava zirconia on Z100 substrate, are fabricated as controls toassure validity of comparisons with previous data.

Example 6

GZG Test configuration. Cracks are initiated and propagated by loadingthe bilayer specimens (a G/Z/G layer of 1.2 mm or 0.4 mm thick onpolycarbonate substrate) with a 3.18 mm radius sphere (tungsten carbideor glass). Initiation and evolution of the near-contact occlusal surfacedamages, namely outer cone, inner cone and median cracks, arecharacterized using a confocal optical microscopy (K2S-BIO, TechnicalInstruments, Burlingame, Calif.), viewing from the contact surface(occlusal surface) and progressively focusing down to the interior ofthe specimen. A cyclic loading fatigue test is performed on amouth-motion simulator (Elf 3300, EnduraTEC, Minnetonka, Minn.) using acontrolled loading profile to simulate the normal chewing function:maximum load (biting force), loading and unloading rates 1000 N/sec, andchewing frequency 1 Hz. Conventional cyclic fatigue profile (fatigue tofailure with a prescribed maximum load) and step-stress fatigue testing(fatigue to failure with consecutively increasing loads) is employed toconstruct the design maps and to predict the performance, respectively.Fatigue testing is interrupted after each cyclic loading step and damagesustained in the G/Z/G layer is examined by confocal microscopy. Testsmay be continued until failure of the G/Z/G layer. Failure is definedwhen one of the near-contact surface crack systems propagates throughthe entire G/Z/G layer or the cementation radial fracture is observed.Occasionally, specimens may be randomly selected and sectioned forcross-sectional examination using the optical microscopy and SEM toconfirm the confocal microscopy observation.

Using transparent polycarbonate as a substrate, initiation of thefar-field radial cracks at the bottom surface of the G/Z/G layer may beimaged directly from below. This provides insight into relative order offracture initiation for competing fracture modes as well as informationabout the load and the number of cycles at which near-contact surfacecracks completely penetrate the specimen and the extent of anydelamination that may develop at the ceramic-epoxy interface. For theinternal surface radial cracks, failure is defined when the crackspop-in because the radial cracks are several millimeters in size whichis sufficient to cause the dental prostheses failure. Weibull statisticsmay be used for data analysis (the current standard method in materialstesting). No overlap of the 90% confidence bound is considered assignificant.

Example 7

Fractography. The fracture surface of randomly selected G/Z/G isanalyzed to determine the effect of a glassy phase on crack path in thegraded Y-TZP at grain level. A thin layer of carbon is deposited on thefracture surface at a 90° incident angle using a carbon coating unit(EMITECH, K250). The fracture surface is examined in an environmentalSEM (Hitachi 3500N) equipped with an energy-dispersive spectroscopy (PGTIMIX) and a backscatter electron imaging detector. Both secondary andback scattered electron imaging modes are utilized to reveal thecrack-microstructure interaction. For comparison, crack paths inhomogeneous Y-TZP ceramics are examined. In addition, controlled cracksand damage are produced in glass-zirconia graded layers and the denseY-TZP layer using both Vickers and Hertizan indentations. Cracktip-microstructure interaction and quasiplastic deformation of gradedstructures are investigated compared to homogeneous Y-TZP. (Guiberteauet al. (1993) Philosophical Magazine A 68(5): 1003-16; Guiberteau et al.(1994) Journal of the American Ceramic Society 77(7): 1825-31; Cai etal. (1994) Journal of Materials Research 9(3): 762-70; Zhang et al.(2003) Journal of Materials Science 38(6): 1359-64).

While the present invention has been set forth in terms of specificembodiments thereof, it will be understood that numerous variations uponthe invention are now enabled to those skilled in the art, whichvariations yet reside within the scope of the present teachings.Accordingly the invention is to be broadly construed and limited only bythe scope and spirit of the present disclosure.

1. A method for preparing a functionally graded glass/zirconia/glass(G/Z/G) sandwich material comprising: (a) applying a powderedglass-ceramic composition to accessible surfaces of a presinteredzirconia substrate thereby covering the zirconia substrate surfaces witha layer of the composition wherein the CTE of the glass and thesubstrate material are substantially the same; (b) infiltrating theglass-ceramic composition into the substrate; and (c) densifying thesubstrate by heating said substrate.
 2. A method in accordance withclaim 1, wherein the substrate comprises yttria-tetragonal zirconiapolycrystal (Y-TZP).
 3. A method in accordance with claim 1, wherein thesubstrate is presintered at a temperature of 900° C. to 1400° C.
 4. Amethod in accordance with claim 1, wherein said densifying is performedin a single firing cycle at a temperature of 1200° C. to 1550° C.
 5. Amethod in accordance with claim 1, wherein the powdered glass-ceramiccomposition is dispersed in an aqueous based solution.
 6. A method inaccordance with claim 1, wherein the powdered glass-ceramic compositioncomprises one or more oxides selected from the group consisting of SiO₂,Al₂O₃, K₂O, Na₂O, BaO, Tb₄O₇, and CaO.
 7. A method in accordance withclaim 1, wherein the CTE of the glass-ceramic is approximately 10.3in/in/° C., from 0 to 430° C., and the CTE of the zirconia isapproximately 10.3 in/in/° C., from 0 to 430° C.
 8. A method inaccordance with claim 1, wherein the presintered zirconia substrate ispresintered by a microwave technique.
 9. A method in accordance withclaim 1, wherein the glass/zirconia/glass (G/Z/G) sandwich material isdensified by a microwave technique.
 10. (canceled)
 11. (canceled) 12.(canceled)
 13. (canceled)
 14. (canceled)
 15. (canceled)
 16. (canceled)17. (canceled)
 18. A functionally graded glass/zirconia/glass (G/Z/G)sandwich material produced by the method of claim 1.